1) The structure of the monoclonal antibody, HyHEL-5 Fab, complexed with its antigen, lysozyme, has been determined in three- dimensions by X-ray diffraction analysis. This structure was now been almost completely refined in two crystal forms. 2) The structure of the monoclonal antibody HyHEL-10 Fab has also been determined and almost completely refined. A comparison of these structures together with a third such complex determined elsewhere has enabled us to make some general statements about antibody antigen interactions.